1. Field of the Invention
This invention relates to unit fuel injectors for internal combustion engines and, in particular, to an electromagnetic unit fuel injector.
2. Prior Art
There are known fuel injection systems wherein the fuel metering and the injection timing occurs by solenoid valves controlling the timing of spill ports closing and opening. One of the difficulties associated with such systems is related to solenoid-to-solenoid scatter in activation and deactivation delays. This leads to unequal duration of closed spill ports in different solenoid valves and may result in poor port-to-port fuel distribution. The higher the injection rate, the more pronounced is this effect on fuel distribution. Since diesel engines utilize high fuel injection rates, application of such an electromagnetic fuel injection to diesels is particularly difficult.
An important item in such a fuel injection system is a solenoid valve in which the air gap between the solenoid core and the armature can be adjusted. A change in the air gap has a pronounced effect on both the activation time and the deactivation time. An increase in the air gap increases the activation time, due to a reduction in the magnitude of the initial magnetic force and an increase in the valve travel distance. It also decreases the deactivation time, due to reduced residual magnetism. Since the valve closed duration is determined by the duration of the solenoid activation pulse less the activation time and plus the deactivation time, an increase in the air gap leads to a decrease in the valve closed duration and, thus, decreases the fuel delivery. Conversely, a decrease in the air gap increases the fuel delivery. With sufficiently fine and precise means of air gap adjustment, the solenoid valves can be calibrated to a specified fuel delivery at a given duration of solenoid activation pulse with high degree of accuracy.
A conventional single thread adjustment is a most widely used devices for adjusting the air gaps in solenoids. The accuracy with which such an adjustment can be performed depends on the pitch of the thread which, due to physical limitations, cannot be made excessively fine.
U.S. Pat. No. 4,232,830 to Casey et al teaches a solenoid valve structure which includes an adjustment screw threaded into an internal bore of a core member to provide adjustment of the valve closure force by means of a pin moving against a spherical ball member. The core member is in turn provided with a threaded end which threads into the rear end cap. The outer thread of the core member serves to adjust the air gap. The adjustment screw provides adjustment of the valve closure force. As a result, these two threads perform two distinctly different adjustments which are not and cannot be performed simultaneously.
U.S. Pat. No. 3,797,756 to Voit et al teaches an electromagnetically actuated fuel injector valve for use with diesel engines. Two threads are also taught in this patent but neither of the two threads is used for adjustments. An outer thread is used for fastening purposes. It helps to secure two parts together by a nut. The inner thread in the bore in the part having an outer thread serves for connection to a return line.
U.S. Pat. No. 3,596,507 to Oshima et al teaches a double-threaded screw arrangement. However, the double threads are not used in combination to provide an adjustment. The thread on one screw is used to adjust the preload of a spring. The outer thread on the part receiving the screw serves to fasten this part to the injector.
The above cited prior art teaches conventional single thread mechanisms each acting individually to perform an adjustment function. It would be desirable to obtain an exceptionally fine adjustment in fuel injectors, typically unattainable with conventional single thread mechanisms.